It's more of a capability thing. If you're running, say, Piston cloud you're using ceph over ethernet to back your disks, so you can easily decouple disk usage and ram usage. If you're stuck using local disks (ie. rackspace/joyent/linode/amazon to a point/etc.), then it's a lot harder to provide that sort of product.

That being said there are providers out there that sell it, and have been for years.